1. Field of the Invention
This invention relates to electrical power generation utilizing the principles of molecular hydrodynamics and the fixation of nitrogen compounds for the subsequent production of nitric acid therefrom. More particularly, the invention relates to magneto hydrodynamic (MHD) generation of electricity using the producer gas stream of a coal gasification process, and the fixation of nitrogen oxides contained therein.
2. Description of the Prior Art
Because of the scarcity of fossil fuels and the increased costs in mining and obtaining these fossil fuels, there is interest in new more economical methods of generating electric power. Among these methods, which have been under investigation, are magneto hydrodynamics (MHD). The process of MHD power generation involves passing a hot gas at an elevated temperature through a magnetic field to generate electric power. At a high temperature, the gas is conductive and ionizable by seeding it with one or more of a plurality of materials such as alkali metal salts. After the gas is passed through the MHD generator, its velocity is reduced and it is cooled, usually with cooling water which is heated by the gas and converted to steam to be further used in the generation of power with conventional steam turbines.
One type of gas used in such MHD power generation is combustion gas. A carbonaceous fuel is combusted to form a hot combustion gas sufficient in temperature to become conductive. Such a combustion gas is termed a plasma. Combustion implies oxidation and therefore an oxidizing media is required, usually air or oxygen enriched air, to combine with the carbonaceous fuel to form the products of combustion. To produce the higher temperatures of combustion required to render the combustion gas conductive, and thus a plasma, a stoichiometric balance is required between the fuel and the oxidizing medium to form a total combustion of the carbonaceous molecules. Theoretically this dictates the addition of just enough oxidizer to completely oxidize the carbon. Practically, a modest overbundance of oxidizer is normally supplied to insure complete combustion. Too much oxidizer will decrease the heat of combustion. A balance must be struck.
During the process of combustion of carbonaceous fuels nitrogen is freed, both from the fuel and extracted from the natural makeup of air. The nitrogen, however, tends to combine at elevated temperatures with free oxygen to form nitrogen oxides. Recently, it has come to be commonly understood that nitrogen oxides in the atmosphere are harmful, even in minute amount, to living organisms. Thus the trend is toward minimizing the exhaust of nitrogen oxides into the atmosphere. One way of doing this is to decrease the level of oxidation by carrying out the combustion in a fuel rich atmosphere, thus reducing the amount of free oxygen resulting in the formation of nitrogen oxides.
Another approach is to collect the nitrogen oxides which are formed before they escape into the atmosphere. This is a difficult and costly process to the extent of being burdensome of carried out as an end in itself. However, if the nitrogen oxides can be readily converted into usable commodities on an economically feasible scale, not only is the project economically possible but it becomes profitable. One of the key factors in profitability is to be able to generate sufficient quantities of nitrogen oxides to supply a conversion plant pursuant to commercial viability. Thus, the approach becomes one of increasing the formation of nitrogen oxides rather than decreasing them.
An MHD generator happens to be a device that readily lends itself to an enhancement of the formation of commercially usable nitrogen oxides as an auxiliary side effect to the production of electric energy.
U.S. Pat. No. 1,443,091 to Petersen teaches the process of generating electricity by means of flowing ionized gas through a magnetic field in an MHD generator to form nitrogen oxides. In addition, Petersen U.S. Pat. No. 1,443,091 teaches increasing the temperature of the gas within the MHD generator to form nitrogen oxides. This temperature increase is to be accomplished by concentrating the direction of the electric current. Petersen U.S. Pat. No. 1,443,091 also teaches using excess air or decomposing carbonic acid or steam to produce "...the oxygen necessary for the production of nitric oxide...", column 4, line 61-62. And further, Petersen U.S. Pat. No. 1,443,091 teaches conversion of the produced nitrogen oxides to, inter alia, nitric acid.
U.S. Pat. No. 3,439,196 to Hals adds to the teachings of Petersen U.S. Pat. no. 1,443,091 the concept of rapid cooling of the gas to fix the nitrogen oxides and that such cooling can be accomplished by rapidly expanding the gas. Hals U.S. Pat. No. 3,439,196 also teaches adding between 50% to 100% excess oxygen to a stoichiometric mixture of fuel gas and oxygen, prior to combustion, to increase the quantity of nitrogen oxides formed even though the temperature of the plasma gas resulting from that combustion will be lowered and, thus, less efficient in producing electricity when passed through the MHD generator. To overcome this decreased efficiency, Hals U.S. Pat. No. 3,439,196 discloses a method of increasing the temperature of the plasma gas by channeling it through an electric arc prior to introduction of that plasma into the MHD generator. The energy to operate the electric arc is to be produced by the MHD generator. Hals U.S. Pat. No. 3,439,196 further teaches the necessity of using a diffuser to reduce the temperature of the plasma stream, after exit from the MHD generator, to reduce the temperature at a rate rapid enough to fix substantial quantities of nitrogen oxides, and, that to accomplish such fixation, the plasma must enter the MHD generator at a supersonic velocity. Finally, Hals U.S. Pat. No. 3,439,196 teaches seeding plasma with an alkali metal to increase ionization.
U.S. Pat. No. 3,471,723 to Maslan states the inference, made in Hals U.S. Pat. No. 3,439,196 that the higher the temperature, the more nitrogen oxides are formed and, thus, the higher the concentration of nitrogen oxides in the plasma gas. Maslan U.S. Pat. No. 3,471,723 discloses simply a method of heating air to a high temperature to produce appreciable quantities of nitrogen oxides. The heat is imposed by channelling the air, already preheated, through an electric arc, the energy for which is produced from an MHD generator through which the superheated air is passed. Maslan U.S. Pat. No. 3,471,723 teaches that a separate expansion nozzle is not necessary, as a great degree of expansion can take place within the MHD generator itself, thus the temperature of the plasma is reduced, by the combination of the MHD generator and a diffuser, at a rapid enough rate to fix nitrogen oxides contained in the plasma. Maslan U.S. Pat. No. 3,471,723 does teach the use of a diffuser, but not as a sole means to cool the gas as in Hals U.S. Pat. No. 3,439,196. Rather, the diffuser is used as both a velocity reducer and a supplemental cooler for plasma exiting the MHD generator. However, it is clear that a temperature below that of the decomposition of nitrogen oxides is not achieved until the plasma is passed through the diffuser. (See Maslan U.S. Pat. No. 3,471,723, Col. 5, line 70, to Col. 6, line 3.) Further, Maslan U.S. Pat. No. 3,471,723 does not teach the use of combustion gases in the system, but rather, directs one skilled in the art away from such gases (see Maslan U.S. Pat. No. 3,471,723, Column 4, lines 54-67). Finally, Maslan U.S. Pat. No. 3,471,723, like Hals U.S. Pat. No. 3,439,196, teaches seeding the gas with alkali metals or their salts.
U.S. Pat. No. 3,546,499 to Somers adds to the prior art the suggestion that the abstraction of electrical energy from the plasma gas passing through a MHD generator, by itself, cools the gas to some degree. The essence of the disclosure of Somers U.S. Pat. No. 3,546,499 is novel: a method and means of cooling the plasma gas, after it exits the MHD generator, in a water spray diffuser, to fix therein nitrogen oxides. Somers U.S. Pat. No. 3,546,499, like Hals U.S. Pat. No. 3,439,196, teaches the use of high temperature combustion gases, enriched with oxygen to form nitrogen oxides, for a plasma directed into an MHD generator. Like Hals U.S. Pat. No. 3,439,196, Somers U.S. Pat. No. 3,546,499 also teaches the need for rapid cooling of the plasma to fix the formed nitrogen oxides. And like Hals U.S. Pat. No. 3,439,196, Somers U.S. Pat. No. 3,546,499 teaches seeding the plasma to increase ionization.
To summarize, the above discussed prior art teaches that gases of combustion can be used to generate electricity by passing them through an MHD generator, that the combustion gas, or plasma, can be further ionized by increasing its temperature and seeding it with an alkali metal or its salt, that by adding excess oxygen to the plasma gas and further increasing its temperature, that nitrogen oxides are formed, that to recover the nitrogen oxides as such, they must be fixed by rapidly cooling the plasma, that a partial cooling can be brought about by rapidly expanding the plasma within the MHD generator, and that to completely fix the nitrogen oxides some means of further cooling the gas beyond the MHD generator is practically necessary.
The most plentiful carbonaceous fuel available is coal. Therefore, it is natural to contemplate coal as a carbonaceous fuel to be gasified in the production of plasma. The current state of the art regarding the gasification of coal is treated in a recent article by Harry Perry entitled, "The Gasification of Coal" published in SCIENTIFIC AMERCIAN Volume 230, Number 3 at page 19 et seq., in March 1974. Of particular interest is the gasification method described therein as the "Koppers-Totzek Process," a well-known method to those skilled in the art. This process is fully described in a treatise "The Production of Gas From Coal Through a Commercially Proven Process" published by Koppers Company, Inc. of Pittsburgh, Pa. 15219 in August 1973.
The K-T Process (Koppers-Totzek) employs the partial oxidation of pulverized coal in suspension with oxygen and steam. The gasifier is a refractory lined steel shell equipped with a steam jacket for producing low-pressure process steam. A two-headed gasifier may be used which is capable of gasifying over 400 tons of coal per day. Coal, oxygen and steam are brought together in opposing gasifier burner heads spaced 180.degree. apart. A four-headed gasifier, capable of gasifying 850 tons of coal per day, employing burner heads 90.degree. apart, may also be used. The coal is gasified almost completely and instantaneously, reacting in the gasifier at a slight positive pressure and at about 3300.degree. F. Carbon conversion is a function of the reactivity of coal, approaching 100 percent for lignites.
Gaseous and vaporous hydrocarbons emanating from the coal at medium temperatures are passed through a zone of very high temperature in which they react so rapidly that coagulation of coal particles during the plastic stage does not occur. The carbon and volitile matter of the coal are gasified, and the coal ash is converted into molten slag. Thus, any coal can be gasified irrespective of coking property, ash content or ash fusion temperature.
As a result of the endothermic gasification reactions and radiation to the refractory walls, the exit gas temperature is decreased to a temperature range exemplified by 2750.degree. F. At the prevailing operating temperatures, only gaseous products are produced. No tars, condensable hydrocarbons or phenols are formed and the process is essentially pollution-free.
Approximately 50 percent of the coal ash drops out as slag into a slag quench tank below the gasifier. The remaining ash is carried out of the gasifier as fine fly ash. Flux can be added to the coal feed to adjust the ash fusion temperature characteristics.
The gasified coal is bled off from the gasifier, at a rate by which equilibrium pressure is maintained within the gasifier, to be used as a combustion fuel.
U.S. Pat. No. 3,916,390 to Moore teaches that a low Btu producer gas can be produced by contacting steam and coal with an oxygen containing gas such as air at a temperature of about 800.degree. to 950.degree. C. (1472.degree. to 1742.degree. F.) Moore U.S. Pat. No. 3,916,390 teaches that such producer gas also contains hydrogen sulfide and carbonyl sulfide. The essence of the invention of Moore U.S. Pat. No. 3,916,390 is a method of stripping the sulfides out of that producer gas. This is done by flowing the sulfide contaminated producer gas through a molten ternary eutectic composition composed of lithium carbonate, sodium carbonate and potassium carbonate to which calcium carbonate is added to extend the temperature range in which the ternary eutectic is functional in respect to cleaning. Since the producer gas manufactured by the process described in Moore U.S. Pat. No. 3,916,390 does not approach the minimum required to generate electricity by way of an MHD generator (Somers U.S. Pat. No. 3,546,499 teaches the minimum required plasma temperature to be 3800.degree. F. at Column 1, lines 49-50), the teachings in relation to MHD generators can only be taken to disclose a method of stripping sulfides from a combustion gas.
To further summarize, the prior art reveals a commercially viable method and apparatus for low pressure gasification of coal along with a method by which sulfides can be stripped from the gasified coal by passing the gasified coal through a melt of alkali metals. The implication is then clear that the stripped gasified coal could be used to produce electric power by MHD generator means with after recovery of nitrogen oxides.
However, major problems exist, the first of which deals with the ash inclusions in the gasified coal. Ash prevents the maximization of the ionization of plasma by the fact that it is electrically inert. The ash also tends to build up on the interior surfaces of the MHD duct.
Sulfur presents a second problem. The alkali metal seeding materials used to increase ionization have a high affinity for sulfur, thus a ready reaction to form alkali sulfur compounds. It is difficult and costly to extract the alkali metals from the sulfur for reuse. Further, the degree of heat required for MHD power generation is far above the point of vaporization of alkali metals, thus a bath of molten alkali metals is unusable to strip sulfur.
The third problem is found in the use of gasified coal. Gasified coal inherently includes dissociated water vapor in the form of H.sub.2. H.sub.2 is about the lightest possible elemental compound in terms of molecular weight. MHD power generation depends on ionization of chemical compounds and the potential for ionization depends to an extent on the molecular weight of the molecules extrapolated by their valences. Inclusion of H.sub.2 excludes higher weight molecules of other compounds and thus diminishes the efficiency of MHD energy generation.
The fourth problem is found in the use of diffusers downstream from the MHD generator. The nature of diffusers is such that they tend to re-compress gases thus increasing their temperature. Thus, the potential exists for a second dissociation of the once fixed nitrogen oxides. If the diffuser operates at theoretical efficiency, heat transfer will more than compensate for the tendency to increase temperature. But, when coal gasification ash is passed through a diffuser, it will adhere to its walls and greatly decrease the efficiency of heat transfer.
The fifth problem is that power is consumed to move the plasma through the MHD generator at a velocity sufficient to generate sufficient quantities of electrical energy. This is compounded by the fact that the prior art systems also tend to use energy produced by the MHD system to preheat the plasma to usable temperatures. Both of these power drains decrease the external efficiency of the system and its potential for producing independently usable energy.
The present invention provides a solution to these problems by presenting an integrated system by eliminating the causes. Means are provided by which ash never reaches the MHD generator duct, sulfur is removed from the plasma before seeding material is introduced, H.sub.2 is substantially removed from the plasma before it reaches the MHD generator duct, nitrogen oxides are completely fixed before exit from the MHD generator duct, the plasma is self-compressed and heated without imposition of external energy or drain of energy produced by the MHD generation system.